The term epitaxial is derived from Greek, meaning to build upon. Epitaxial deposition, in general, is a deposition of a layer of single-crystal silicon on a single-crystal wafer. The deposited layer is a crystallographic extension of the substrate in terms of "seed" that is necessary to promote the single-crystal deposition.
Epitaxial deposition is a chemical vapor deposition (CVD) process. The original use of CVD started with the deposition of a single-crystal silicon in the late 1950's and has played a major role in the industry since.
Epitaxy (or epi) has played a major role in the evolution of bipolar transistors and bipolar integrated circuits. In recent years both MOS discrete transistors and ICs have started to use epi as a key part of their structures.
Silicon CVD processes can be categorized by temperature range, pressure, and reactor wall temperature. There are two types of Silicon CVE processes. Namely, Epitaxy, which follows substrate crystal orientation, and Polysilicon, having no crystal orientation on amorphous layers. The present invention involves the use of the latter with plasma enhanced chemical vapor deposition devices, or PECVD. Although other CVD processes can easily be adapted to use the disclosed embodiment.
Thin-film polysilicon deposition has many important applications in the semiconductor industry. Polysilicon was the key ingredient leading to the self-aligned MOS technology. Heavily-doped polysilicon has become the most widely used gate-electrode material for MOS products, both discrete and ICs. In addition, it serves as an interconnect, capacitor plate, doping source, and can be oxidized to form a stable layer of SiO.sub.2. Polysilicon is utilized in these roles because of its compatibility with subsequent high-temperature processing, its excellent interface with SiO.sub.2, its high reliability as a gate electrode material, and its ability to be deposited over steep topography with good conformal coverage. Lightly-doped polysilicon films are used as resistors in static memory products and to fill trenches in DRAMS. Thin films of polysilicon are made upon of small single-crystal grains of about 1000 angstrom separated by grain boundaries. The film that will be a polysilicon layer con be either amorphous or polycrystalline as deposited. Subsequent heat cycles at elevated temperatures will cause an amorphous film to become polycrystalline. This as deposited undoped film has an extremely high resistivity.
Over the years several types of CVD processes have evolved. Plasma-Enhance CVD (PECVD) reactors emerged in the late `80`s and early `90`s. PECVD systems can be operated at lower temperatures making item attractive for submicron technology.
One such PECVD machine is broadly illustrated in FIG. 1, with the following elements: PECVD 10 has chamber walls 12, door 14, wafers 16, heater 20, plasma gas applicators 18, PECVD chamber 22, bottom 24 and upper 30 plates for wedging the wafer holding arms 26, and locking screws 28 that hold the plates together.
FIGS. 2 and 3 illustrate an isolated view of top views of the upper plate 30 and lower plate 24 having the following elements: a pair of parallel wafer arm chambers 32, wafer arm holder spring lock chambers 30, wafer holding arms (represented by the single pair) 31 would be wedged between plates 30 and 24 in the chambers 32, where a spring (not shown in its chamber 30) would wedge the two arms in their chambers 32. It is noted that there are eight pairs of arms 31. Seven are deposition stations, and the last is for wafer loading and unloading. The device of FIGS. 1-3 can be purchased from Novellus, at 81 Vista Montana, San Hose, Calif. 95134; model no. 02-00163-00, described as Assay, fork ceramic, other related numbers are identified for parts of a complete PECVD.